1. Field of the Invention
The present invention relates to a sheet supplying apparatus for separating and supplying a sheet one by one from a sheet stack, and more particularly, it relates to a structure of a sheet containing portion removably mountable in a sheet feeder portion.
2. Related Background Art
Generally, in sheet supplying apparatuses used with copying machines, printers, facsimiles and the like, a number of sheets (such as transfer sheets, photosensitive sheets and the like) are stacked on a sheet receiving plate of a cassette or deck, and such sheet is separated and supplied one by one from the stacked sheets (sheet stack) by means of a sheet supply means such as sheet supply rollers and the like and is fed toward a next processing station. In this case, to prevent a so-called double-feed, i.e., the fact that two or more sheets are supplied at a time, the provision of separating pawls is already known. More particularly, separating pawls are arranged at a leading end of the sheet stack with respect to a sheet supplying direction, and, when an uppermost sheet is supplied, it rides over the separating pawls while forming a loop at a leading end portion of the uppermost sheet, whereby the uppermost sheet is separated from the other sheets, with the result that only one sheet is supplied.
FIG. 29 is a perspective view of a main portion of an exemplary sheet supplying apparatus having separating pawls. In FIG. 29, the reference numeral 100 denotes a sheet stacking support (intermediate plate) acting as a sheet receiving plate; 101 denotes springs for biasing the sheet stacking support 100 upwardly; P denotes a sheet stack comprised of sheets (cut sheets or papers) having the same size; 102 denotes sheet supply rollers; and 103 denotes a pair of left and right separating pawls disposed on and engaged by front left and right upper corners of the sheet stack P with respect to a sheet supplying direction. An upper surface of the front or leading end portion of the sheet stack P is urged against lower surfaces of the sheet supply rollers 102 with a predetermined pressure by lifting the sheet stacking support by means of the springs 101. Alternatively, the sheet supply rollers 102 may be lowered to urge against the upper surface of the sheet stack P in response to a respective sheet supply signal. Each separating pawl 103 is pivotally mounted on a pin 103a for movement in an up-and-down direction so that the pawl is rested on the corresponding front corner of the sheet stack P by its own weight.
When the sheet supply rollers 102 are rotated in the sheet supplying direction, an uppermost sheet P1 of the sheet stack P is subjected to a feeding force directing toward the sheet supplying direction by the friction force between it and the sheet supply rollers 102. Thus, the uppermost sheet P1 tries to advance in the sheet supplying direction; however, since the left and right front corners of the sheet are restrained by the separating pawls 103, the uppermost sheet cannot advance in the sheet supplying direction. As a result, as the sheet supply rollers 102 are rotated, a bent loop E is formed in the uppermost sheet P1 near the separating pawls 103 between the sheet supply rollers 102 and the separating pawls 103 in opposition to the resiliency of the sheet P1. As a result, when the bent loop E grows up to a certain extent, by a restoring force tending to return the bent loop E to the original state, the left and right front corners (retained by the separating pawls 103) of the uppermost sheet P1 naturally shift from lower surface sides to upper surface sides of the separating pawls 103, thus riding over the separating pawls 103. That is to say, by forming and growing the bent loop E in the uppermost sheet P1, the latter is released from the separating pawls 103, with the result that only the uppermost sheet is separated from the other sheets P.
By the way, pursuant to the variety of information and the increase in information dissemination, the amount of sheets to be used has increased. Thus, in order to eliminate the trouble regarding the replenishment of sheets, the sheet stacking ability of the sheet supply cassette has been increased, for example, from 250 sheets to 500 sheets. However, in the above-mentioned sheet supplying apparatus, it was feared that, when the increased sheets were stacked on the intermediate plate 100, as the uppermost sheet of the sheet stack P was separated from the other sheets and fed out by the sheet supply rollers 102, the left and right front corners of the uppermost sheet could not be disengaged from the separating pawls 103, thus causing the poor sheet supply.
That is to say, despite the fact that the smooth supply of the uppermost sheet is greatly influenced upon the urging force of the sheet supply rollers 102 against the uppermost sheet P1, i.e., the lifting force for lifting the intermediate plate 100 (which lifting force relates to the urging force), such lifting force is exclusively dependent upon the feature of the springs 101. Since the proper sheet supply can be affected by the delicate balance between the lifting force against the intermediate plate 100 and the weight of the sheets and of the intermediate plate 100, even if the number of sheets to be stacked is increased without changing the fundamental construction of the sheet supply cassette, abnormal sheet supply will occur frequently.